Method and apparatus for facilitating reception of a signal on one of a plurality of contiguous channels

Abstract

In a zero intermediate frequency (ZIF) receiver ( 300 ) a desired signal is detected ( 902 ) and downmixed ( 924 ) to baseband, off-centered by an amount determined by at least one of (a) whether ( 906 ) pilot symbols are present in the desired signal, and (b) signal levels ( 908 ) of the desired signal and adjacent channel signals.

What is claimed is:  
     
       1. A method in a zero intermediate frequency (ZIF) receiver for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference, the method comprising the steps of: 
       detecting the desired signal; and  
       downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal, wherein the amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.  
     
     
       2. The method of  claim 1 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the downmixing step comprises the step of off-centering the downmixed signal by about one-half the pilot symbol rate.  
     
     
       3. The method of  claim 1 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the method further comprises the step of measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels, and  
       wherein the downmixing step comprises the step of off-centering the downmixed signal by about one-half the pilot symbol rate, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       4. The method of  claim 1 , 
       wherein the desired signal does not include the pilot symbols, and  
       wherein the method further comprises the step of measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels,  
       wherein the downmixing step comprises the step of centering the downmixed signal at zero Hz, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       5. The method of  claim 1 , 
       wherein the plurality of channels are separated in frequency by a channel spacing, and  
       wherein the method further comprises the step of measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels, and  
       wherein the downmixing step comprises the step of off-centering the downmixed signal by one-half the channel spacing, when at least one of a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level does not exceed a threshold.  
     
     
       6. The method of  claim 1 , further comprising the step of 
       measuring signal levels on two channels adjacent to the desired channel to determine which adjacent channel has a higher signal level,  
       wherein the downmixing step comprises the step of off-centering the downmixed signal in a direction such that the adjacent channel having the higher signal level is shifted in frequency away from zero Hz.  
     
     
       7. An apparatus in a zero intermediate frequency (ZIF) receiver for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference, the apparatus comprising: 
       means for detecting the desired signal; and  
       means for downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal, wherein the amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.  
     
     
       8. The apparatus of  claim 7 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the apparatus further comprises means for off-centering the downmixed signal by about one-half the pilot symbol rate.  
     
     
       9. The apparatus of  claim 7 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the apparatus further comprises:  
       means for measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       means for off-centering the downmixed signal by about one-half the pilot symbol rate, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       10. The apparatus of  claim 7 , 
       wherein the desired signal does not include the pilot symbols, and  
       wherein the apparatus further comprises:  
       means for measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       means for centering the downmixed signal at zero Hz, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       11. The apparatus of  claim 7 , 
       wherein the plurality of channels are separated in frequency by a channel spacing, and  
       wherein the apparatus further comprises:  
       means for measuring a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       means for off-centering the downmixed signal by one-half the channel spacing, when at least one of a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level does not exceed a threshold.  
     
     
       12. The apparatus of  claim 7 , further comprising: 
       means for measuring signal levels on two channels adjacent to the desired channel to determine which adjacent channel has a higher signal level; and  
       means for off-centering the downmixed signal in a direction such that the adjacent channel having the higher signal level is shifted in frequency away from zero Hz.  
     
     
       13. A subscriber unit for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference, the subscriber unit comprising: 
       a receiver front end for detecting the desired signal;  
       a processing system coupled to the receiver front end for processing the desired signal and for controlling the subscriber unit; and  
       a downmixer coupled to the receiver front end and comprising a portion of the processing system for downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal, wherein the amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.  
     
     
       14. The subscriber unit of  claim 13 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the processing system is programmed to off-center the downmixed signal by about one-half the pilot symbol rate.  
     
     
       15. The subscriber unit of  claim 13 , 
       wherein the desired signal includes the pilot symbols inserted at a predetermined pilot symbol rate, and  
       wherein the processing system is programmed to:  
       cooperate with the receiver front end to measure a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       off-center the downmixed signal by about one-half the pilot symbol rate, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       16. The subscriber unit of  claim 13 , 
       wherein the desired signal does not include the pilot symbols, and  
       wherein the processing system is programmed to:  
       cooperate with the receiver front end to measure a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       center the downmixed signal at zero Hz, when both a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level exceed a threshold.  
     
     
       17. The subscriber unit of  claim 13 , 
       wherein the plurality of channels are separated in frequency by a channel spacing, and  
       wherein the processing system is programmed to:  
       cooperate with the receiver front end to measure a desired signal level on the desired channel and first and second signal levels on first and second adjacent channels; and  
       off-center the downmixed signal by one-half the channel spacing, when at least one of a ratio of the first signal level to the desired signal level and a ratio of the second signal level to the desired signal level does not exceed a threshold.  
     
     
       18. The subscriber unit of  claim 13 , wherein the processing system is programmed to: 
       measure signal levels on two channels adjacent to the desired channel to determine which adjacent channel has a higher signal level; and  
       off-center the downmixed signal in a direction such that the adjacent channel having the higher signal level is shifted in frequency away from zero Hz.